Traveling-wave tube



March 29, 1960 s. SENSIPER TRAVELING-WAVE TUBE Filed Sept. 30, 1957 IN VEN r02, SAMUEL SENS/PER,

er [W AGENT Unite States Patent TRAVELING-WAVE TUBE Samuel Sensiper, Los Angeles, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Application September so, 1957, Serial N0. 686,912

Claims. c1. sis-3.5

This invention relates to microwave devices and particularly to traveling-wave tubes incorporating a slowwave'structure of the folded or loaded waveguide type and which have nonreciprocal attenuation characteristics.

Traveling-wave tubes generally employ a slow-wave structure to propogate an electromagnetic wave along the path of an electron stream. The longitudinal velocity of the wave should be such as to effect interaction between the electron stream and the wave, this electromagnetic wave being known as a traveling wave. Slowing down the traveling wave to approximately the velocity of the electron stream in order to secure this interaction is normaily accomplished by a folded waveguide, a helix, a serpentine structure, or one of various other configurations. It is well known that an appropriate electron stream in association with such a structure may deliver energy or push in a manner to cause amplification or gain to microwave signals traveling along the slow-wave structure. The gain may be eifective throughout an exceedingly broad range of frequencies. However, it is very diflicult to provide an output circuit with an impedance that matches that of the slow-wave structure over the operable frequency range. This fact usually results in a portion of the energy of the electromagnetic waves appearing at the output circuit being reflected and propagated back towards the input, low signal level, end of the slow-wave structure. When this reflected portion of the output wave arrives at the input circuit, a portion may again be reflected and propagated, with consequent amplification, towards the output circuit. Such a re-reflection may result in selfoscillation and cause the tube to be inoperable or impractical for operation in the frequency ranges wherein such oscillation occurs.

A common method of precluding such oscillations is to place attenuation means along the length of the slowwave structure so as to absorb the reflected energy and preclude its regenerative build-up. Such means are generally, however, inherently reciprocal in nature and decrease the forward or desired gain of the traveling-wave tube. This sacrifice of forward gain is a practical solution in low-powered tubes, or in other cases where efiiciency of operation is not of prime importance.

However, where maximum gain and efliciency is desired, as in a high-powered tube, a nonreciprocal loss is imperative. In this connection, the phenomenon of ferromagnetic resonance in a biased ferromagnetic dielectric substance, known as a ferrite, has been utilized in traveling-wave tubes to provide a nonreciprocal loss. Inorder to utilize this phenomenon in a waveguide excited conventionally, as in the TE mode of propagation, a fern'te element may be disposed within the waveguide, near and parallel to the shorter height waveguide wall. The elongated element is usually placed in the region in which the radio frequency magnetic field appears circularly polarized. This region of circular polarization is off-center, to one side of the waveguide, for a direction of propagation in one direction in the waveguide, and oif-center to the other side of the waveguide for the opposite direction of in 2,930,921 Patented Mar. 29, 1960 propagation. When a transverse, biasing magnetic field is applied along the elongated ferrite placed in this manner, ferromagnetic gyro-resonance may occur in the ferrite and the incumbent losses elfectively attenuate radio frequency energy traveling in one direction, although substantially not alfecting microwaves traveling in the opposite direction.

Various means have been devised for utilizing this phenomenon in a traveling-wave tube. In a copending application entitled Traveling-Wave Tube, by Samuel Sensiper, Serial Number 473,856, filed December 8, 1954, now Patent No. 2,806,972, and assigned to the assignee of the present invention, there is disclosed one such arrangement. ,In one form of the invention there described are means for utilizing the axial magnetic field provided for focusing and confining the electron stream. A folded waveguide slow-wave configuration is folded back and forth across the electron stream. The arrangement is such that,- in the region of the electron stream and the ferrite slabs there utilized, the direction of propagation of microwaves is perpendicular to the direction of the axial magnetic field. Hence, that field is utilized as well to bias the ferrite slabs for gyro-resonance attenuation. Properties and examples of ferrite materials which may be utilized for gyro-resonance are described in an article entitled Ferrites In Microwave Applications, by J. H. Rowen, pages 1333-1369, BellSystem Technical Journal, November 1953 (New York).

The above-described invention is of great practical benefit, particularly for circuits of simple form. It may be desired, however, to provide other circuits having even higher efficiency and greater forward to backward loss ratios. In such cases, it is recognized that ferrite slabs distort the magnetic fields involved, specifically, the static focusing field and the radio frequency magnetic compo nents. This distortion may impose limitations on the efliciency of many high-powered traveling-wave tube amplifier designs.

I It is therefore an object of this invention to provide an improved traveling-wave tube.

It is another object of the present invention to provide a high-power, high-efficiency traveling-wave tube having improved unidirectional or nonreciprocal attenuation characteristics.

Still another object of this invention is to provide an improved broadband amplifier haviuggreater gain over a wider range of frequencies than has heretofore been possible.

It is another object of the invention to provide such apparatus utilizing ferrites biased by the axial electron stream focusing field.

Briefly, these and other objects are achieved in thepresent invention by a combination of relatively slender ferrite rods and a folded waveguide or loaded waveguide type of slow-wave structure. The ferrite rods are disposed longitudinally that is, axially, along the slow-wave structure and in the region in which the radio frequency field of the undesired reflected portion of the travelingwave is circularly polarized. The ferrite rods are placed on opposite sides of adjacent cells defined by the slowwave structure and interconnected by drift tubes such that microwave energy in alternate cells is traversing the electron stream'in opposite directions. The present invention provides an arrangement which greatly reduces the distorting effects of the relatively low a ferrite rods on the magnetic fields which are employed. Compensating magnets or ferromagnetic elementsare disposed in a region external to the interaction cells. These very high a field distorting members far from the beam are so arranged in the combination as to compensate for the use of the ferrite, lower u, field distorting members close to the provides fields shaped to a uniform value. The high a soft iron or permanent magnet members can be disposed external to the structure, on the internal surface thereof, or imbedded in the wall of the structure.

The novel features of this invention, as well as further objects and advantages of the invention as regards its structure and method of operation, will be more clearly understood when considered in the light of the following description, taken in conjunction with the accompanying drawings, in which like reference numerals refer to like parts, and in which:

Fig. 1 is a schematic view, shown partly in section, of a traveling-wave tube embodying the present invention;

Fig. 2 is a cross-sectional view of the traveling-wave tube of Fig. 1 taken along the section lines 22;

Fig. 3 is a longitudinal section of the tube of Fig. 1 taken along section lines 33; and

Fig. 4 is a longitudinal sectional view, similar to that of Fig. 3, of an alternative embodiment of the present invention.

Referring now with more particularity to the drawings, in Fig. 1 there is shown a traveling-wave tube in which a folded waveguide type of slow-wave structure 12 is utilized for effectively slowing down microwave energy traversing along the longitudinal axis of the tube 10 from left to right (as seen in the figure) through the tube 10. It will be understood by those skilled in the art that the tube shown has been simplified for clarity and is not drawn to scale. The slow-wave structure 12 comprises a series of transverse vanes 14 which alternately project from opposite sides of an enveloping waveguide wall 16. The vanes 14, as well as the waveguide walls 16, are composed of a conductor which is nonmagnetic, such as copper or brass. The vanes 14 are mounted conductively on the waveguide walls 16 at spaced intervals, which spacing thereby provides interconnected interaction cells :17. An interaction cell 17 as utilized here is a volume limited by the planes of two adjacent vanes 14 and the four waveguide walls 16. Through substantially the center of each cell 17 passes an electron beam 19. The velocity of the electrons is approximately that of the electromagnetic energy which is made to propagate the length of the slow-wave structure 12. The electromagnetic energy is axially effectively slowed down by the tortuous or serpentine path formed by the alternating configuration of the vanes 14. The vanes 14 are relieved, as shown, in the area where the electron beam would otherwise intersect with the vanes. Associated with each vane 14 there is a nonmagnetic drift tube 21 secured within the opening for the beam 19. The drift tubes 21 are thus disposed in axial alignment along the length of the slow-wave structure 512 and form the path for the electron beam 19 through and in energy exchange relationship with microwave energy in each of the interaction cells 17. The drift tubes 21 preclude interaction between the election beam and the traveling waves in the cells 17, except in the region of each cell near its center where the traveling wave is in proper phase rela tion with the bunching or density modulation signal on the electron beam 19.

Near the lefthand extremity of the traveling-wave tube 10 is an electron gun 18 which may be comprised of the usual cathode 20, heated by a filament 22 to emit the stream of electrons which is shaped and focused by a focusing electrode 24 into an electron beam 19. An anode 26 accelerates the beam to a high velocity. A voltage source 28 is tapped at appropriate potentials to provide operating voltages to these electrodes 20, 24-, 26. Near the righthand extremity of the tube 10 is a collector electrode 30 which intercepts the stream of electrons and dissipates their kinetic energy.

A source of potential 32 is connected between the collector electrode 30 and ground, making the collector electrode 30 positive with respect to the slow-wave structure 12. Thus, secondarily emitted electrons from the 4 slow-wave structure 12 will not find their way back into the interaction region of the tube 10 to give rise to noise or other interference. Connected to the lefthand extremity, or input end, of the tube 10 is an input waveguide section 34 connected by a flange to external circuitry (not shown). The coupling flange may include a microwave window 36 to thus allow a pressure differential between the exterior and the evacuated interior of the tube 10; similarly, an output waveguide section 38 is connected to the output end of tube 10 where again the coupling flange may include a microwave window 40. Disposed about substantially the length of tube 10 and energized by a source of potential 32 is a focusing solenoid 44 for constraining or confining the electron beam to its axial path along the tube 10.

Disposed in each interaction cell 17 formed by the vanes 14 along slow-wave structure 12 is an elongated ferrite rod 46. The rods 46, as may be seen more clearly from Fig. 3, are disposed on opposite sides of the cells in alternating fashion along the length of the tube 10. The rods 46 are substantially parallel to the axis of the tube 10 and are laterally displaced therefrom. The ferrite rods lie in a plane substantially equidistanut from the side of tube 11'}, represented in Fig. 2 as the top surface of the tube 10. The rods are approximately of the same axial length as each cell 17 and are supported thereacross by being secured, at each end of each rod, to a pair of adjacent vanes 14. The properties of the ferrite material of these rods 46 are described in the above-mentioned article by Rowen in the Bell System Technical Journal. The ferrite rods 46 may, for example, be composed of ferrite material known commercially as Ferramic A, Ferramic G, Ferramic R1, ferroxcube 104 or ferroxcube 106.

The sectional views of Fig. 2 and Fig. 3 illustrate the compensating means of the invention in the form of magnetic material, such as soft iron slabs 48, associated with each interaction cell 17 to balance out or correct the distortion of the magnetic field caused by the inser tion of the ferromagnetic dielectric material of the ferrite rods close to the electron stream as discussed below. The slabs 48 are shown placed on the exterior of waveguide 16 near each cell 17, directly opposite from the respective ferrite rod 46 in the respective cell. The shape and size of slabs 48 is best determined empirically by placing just enough iron on the waveguide wall to minimize the interception of electrons in the beam 19 by the drift tubes 21.

Referring to Fig. 4, an alternative embodiment is shown in which, although the ferrite rods 46 are specified similarly to those of Fig. l-Fig. 3, the magnetic field compensation means are small permanent magnets 50 placed outside the waveguide 16 and associated with each interaction cell 17. Again, the exact size and shape of the magnets is best determined by empirically minimizing electron beam interception by the slow-wave circuit elements of the tube 10.

In the operation of the present invention, microwave energy is impressed upon input waveguide section 34 of tube 10, and is caused to propagate the length of the slowwave structure 12 in an undulating fashion, as is indicated by arrows in Fig. l. The microwave energy thus impressed upon and propagated by the slow-wave structure 12 is emitted from the tube 10 to the output waveguide section 38. The electron gun 18 projects a stream of electrons along the axis of the tube 10, through the drift tubes 42, to the collector electrode 30 which collects the stream electrons and dissipates their energy. An appropriate coolant (not shown) may be forced past the tins of the collector electrode 30 to aid in cooling it.

An axial magnetic field produced by the solenoid 44 is used in a conventional manner to focus the stream of elec trons along the indicated axial path through the drift tubes 42. In accordance with the present invention, however, the same axial magnetic field is also used to bias the nonreciprocal attenuating ferrite elements 46. The ferrite rods 46 are placed in the region of circular polarization of the radio frequency magnetic fields of reflected microwave energy being undesirably propagated in the backward direction (from right to left as viewed in the drawing). To prescribe analytically exactly where the ferrite rods should be placed is exceedingly ditficult because of the aberrations from a plane front wave as the microwave energy radiates around the 180 turns of the slow-Wave structure 12. The placements shown are approximate, however, and may be varied empirically for a specific design and operation to provide optimum results. The use of ferrite rods 46 instead of slabs of ferrite improves the nonreciprocal effect by an order of magnitude due to the concentration of ferrite material in the region of circular polarization. However, the use of the ferrite rods 46 causes a distortion of the axial focusing magnetic field. This distortion is balanced or compensated for by the ferromagnetic material slabs 48, which appear in Figs. 2 and 3, or by the permanent magnets 50, shown in Fig. 4. These compensating members likewise distort the magnetic field, but in a manner to substantially cancel out the distortion due to the ferrite rod combination of high ,1. field distorting members far from the beam and is equivalent to the use of low ,u. field distorting members close to the beam. The result is a shunting of the field to a uniform value. The soft iron members 48 may be placed external to the structure on the wall or imbedded in the wall of the tube 10.

The alternate arrangement shown in Fig. 4 utilizes permanent magnets 50 to oppose the induced leakage field of the ferrite rods 46 and thus return the field to its original mean value rather than to shunt it. As shown by the north-south markings on the magnets 50 and ferrite rods 46 in Fig. 4, the leakage field from the permanent compensating magnets 50 opposes the induced leakage field of the ferrite rods 46 so as to restore the axial field in the region of the beam to its original value.

In a specific, practical example of the present invention, by using soft iron slabs immediately exterior to the waveguide wall 16 in the relative positions shown by the figures, loss ratios of the order of to 1 in decibels have been readily achieved. In the particular tube referred to, the axial length of the interaction cells, that is, the distance between adjacent vanes 14, is .157 inch. The ferrite rods have the same length and are .032 inch in diameter. The outer diameter of the drift tubes is .187 inch and .092 inch in length. The soft iron slabs are .03 inch thick and .078 inch square and are symmetrically disposed on wall 16, as shown in Fig. 2.

There has thus been disclosed a traveling-wave tube utilizing longitudinal ferrite rods which are biased by the longitudinal focusing magnetic field. A very high at-.

tenuations ratio is achieved and defocusing problems caused by distortion effects on the beam by the ferrite rods are solved by high ,a field compensation means disposed in a manner to cancel out the field distortion effects of the ferrite rods.

What is claimed is:

1. A traveling-wave tube comprising means for projecting an electron stream along a predetermined path; magnetic means providing a longitudinal magnetic field for focusing said electron stream; a folded waveguide type slow-wave structure comprising adjacent interconnected interaction cells disposed along said predetermined path; and nonreciprocal ferromagnetic gyroresonance attenuation means including an elongated ferrite rod disposed in one of said cells substantially parallel to said predetermined path and field compensation means disposed in the proximity of said cell in a manner to compensate for magnetic field distortion caused by said ferrite rod.

2. A traveling-wave tube comprising: means for projecting an electron stream along a predetermined path; a folded waveguide type slow-Wave structure for providing electromagnetic energy exchange between microwaves 6 traveling therealong and said electron stream; said slowwave structure being of the form to provide adjacent interconnected interaction cells along said predetermined path for effectively causing said microwave energy to propagate in undulating fashion back and forth across said predetermined path; nonreciprocal attenuation means comprising an elongated ferrite rod disposed in one of said cells substantially parallel to said predetermined path, said ferrite rod being laterally disposed from said path and having a diameter sufliciently small as to lie substantially only in the region of circular polarization of the radio frequency magnetic fields of said microwave, energy propagating in a direction opposite to that of said electron stream whereby ferromagnetic gyro-resonance losses in said ferrite substantially eliminate said backwardly propagating microwave energy while not appreciably affecting microwave energy traversing said slow-wave structure in the direction of projection ofsaid electron stream.

3.A traveling-wave tube comprising means for projecting an electron stream along a predetermined path; slow-wave structure waveguide means disposed about said electron stream in microwave energy exchange relation therewith and including transverse loading vanes extending inwardly from .a pair of oposite walls of said waveguide in an alternating arrangement whereby microwave energy traversing said waveguide slow-wave structure is caused to undulate back and forth across said electron stream; nonreciprocal attenuating means including elongated fcrrite rods disposed parallel to said predetermined path between adjacent pairs of said transverse vanes and particularly disposed in the region of circular polarization of the radio frequency magnetic fields associated with microwave energy traversing said slow-wave structure in a direction opposite to that of said electron stream; magnetic focusing means for providing a biasing magnetic field in said ferrite rods whereby said microwave energy traversing said tube oppositely to said electron stream is unidirectionally attenuated by virtue of ferromagnetic gyro-resonance losses in said ferrite rods.

4. A traveling-wave tube comprising means for projecting an electron stream along a predetermined path; loaded waveguide type cells disposed along said predetermined path, the walls between adjacent ones of said cells being relieved about said predetermined path thus permitting the flow of said electron stream, adjacent ones of said cells being electromagnetically interconnected in a manner whereby microwave energy traversing the traveling-wave tube along said predetermined path is caused effectively to undulate back and forth across said electron stream as it propagates from cell to cell; nonreciprocal attenuation means including elongated ferrite rods supported in said' cells between said walls parallel to said predetermined path and having a diameter and being disposed in a lateral position such that said ferrite rod lies substantially entirely in the region of circular polarization of the radio frequency magnetic fields of microwave energy traversing said traveling-wave tube in a direction opposite to that of said electron stream; magnet means for providing a longitudinal focusing field for said elec tron stream whereby said focusing field biases said ferrite rods in a manner such that said microwave energy traversing in a direction opposite to that of said electron stream is substantially absorbed in ferromagnetic gyroresonance losses in said ferrite while not appreciably affecting microwave energy traveling in the opposite direction; and magnetic field compensating means disposed longitudinally even with said ferrite rods for substantially eliminating perturbations upon said magnetic fields by the presence of said ferrite rods.

5. The invention according to claim 4 in which said compensating means comprises ferromagnetic slabs disposed at a wall of said waveguide diametrically opposite from said ferrite rods.

6. The invention according to claim 4 in which said compensating means includes small permanent magnets disposed longitudinally even with said ferrite rods at the wall of said waveguide diametrically opposite about said predetermined path from respective ones of said ferrite rods.

7. A traveling-wave tube comprising: means for projecting an electron stream along a predetermined path; an elongated waveguide disposed coaxially about and along said path; two sets of transverse loading vanes disposed perpendicularly to and spaced substantially evenly along the length of said waveguide and extending interdigitally from opposite sides thereof toward and beyond said path in a manner thereby to form adjacent interconnected interaction cells, each of said vanes being relieved in the region of its intersection with said stream; an elongated ferrite rod disposed in each of said cells laterally displaced from and substantially parallel to said path, said rods and said path lying substantially equidistant from said opposite sides of said waveguide, adjacent ones of said rods lying on opposite sides of said path and said rods having a length substantially equal to the axial length of its respective cell; and magnet means for providing a longitudinal magnetic field for focusing said electron stream and biasing said ferrite rods.

8. A traveling-wave tube comprising: means for pro jecting an electron stream along a predetermined path; an elongated waveguide disposed coaxially about and along said path; two sets of transverse loading vanes disposed perpendicularly to and spaced substantially evenly along the length of said waveguide and extending interdigitally from opposite sides thereof toward and beyond said path in a manner thereby to form adjacent interconnected interaction cells, each of said vanes being relieved in the region of its intersection with said stream; an elongated ferrite rod disposed in each of said cells laterally displaced from and substantially parallel to said path, said rods and said path lying substantially equidistant from said opposite sides of said waveguide, adjacent ones of said rods lying on opposite sides of said path and said rojds having a length substantially equal to the axial length of its respective cell; magnet means for providing a longitudinal magnetic field for focusing said electron stream and biasing said ferrite rods; and magnetic field compensating means associated with each of said ferrite rods for substantially eliminating perturbations upon said magnetic field caused by the presence of said ferrite rods.

9. The invention according to claim 8 in which said compensating means comprises a ferromagnetic slab disposed in each of said interaction cells at the side of said waveguide diametrically opposite from said ferrite rod associated with that cell.

10. The invention according to claim 9 in which said compensating means includes a small permanent magnet disposed in each interaction cell longitudinally even with the ferrite rod associated with that cell at the side of said waveguide diametrically opposite from said ferrite rod.

References Cited in the file of this patent UNITED STATES PATENTS 2,636,948 Pierce Apr. 28, 1953 2,653,270 Kompfner Sept. 22, 1953 2,766,393 Casey Oct. 9, 1956 2,790,920 Todd Apr. 30, 1957 2,806,972 Sensiper Sept. 17, 1957 2,829,301 Azema Apr 1, 1959 FOREIGN PATENTS 750,208 Great Britain June 13, 1956 

